A wireless telephone is only useful for as long as its battery can provide power. Thus, power efficiency is a dominant factor in determining the overall quality and utility of any wireless communications product. As a result, many innovations have been achieved in the areas of battery technology and intelligent power consumption.
A relatively new innovation, however, adds significant demands upon the battery of a wireless telephone. Namely, the incorporation of a GPS unit into a wireless telephone delivers powerful capabilities to system operators as well as advanced features to end users. Specifically, system operators can employ GPS to facilitate call setup, call routing, billing, and tracking of the telephones. Furthermore, end users are provided with pinpoint navigation services, emergency location services and many other desirable features.
In order to provide such positioning services on demand, a GPS receiver must establish and maintain a heightened state of readiness to support the rapid acquisition and tracking of a plurality of GPS satellites. Ranging measurements derived from tracking these satellites fuel the computation of accurate estimates of position, velocity, and time (PVT).
Following an initial location determination, a GPS receiver maintains this state of readiness by periodically refreshing critical operational data. These data can be divided simply into short-term and long-term GPS information. Short-term GPS data includes the receiver's current estimates of GPS position, velocity, and time, and a running list of visible satellites. Such information is updated by periodically acquiring/reacquiring satellites and computing PVT fixes. These update intervals may range from several seconds to some number of minutes, depending upon the technology employed in the GPS receiver.
Long-term GPS information includes the ephermeris and almanac information broadcast in the navigation messages of the GPS satellites. For a given satellite, the ephermeris data includes highly accurate modeling parameters that describe the orbital path and atomic clock drift of that particular satellite. Ephermeris parameters are updated about every two hours for each visible satellite in order to support accurate PVT computations. The almanac data contains less accurate modeling parameters for the entire GPS constellation. Almanac information is critical to the receiver's startup sequence and in predicting the rise of satellites into the receiver's visible region. Depending upon the implementation, almanac data may only require an update every several days.
As stated above, both short-term and long-term GPS information must be properly refreshed and maintained in order to prepare a receiver to produce prompt and accurate PVT fixes upon system or user demand. With readiness such a critical concern, an elementary approach would be to mechanize the receiver to continuously track GPS satellites and update these data at some very high rate. However, each acquisition, each tracking operation and each demodulation of the navigation data requires an expenditure from the wireless telephone's precious energy reserves.
In a effort to preserve battery energy, it is common practice to operate a GPS receiver in a stand-by mode of readiness wherein the receiver expends energy according to some fixed predetermined schedule of short-term and long-term GPS data maintenance. In addition to adhering to rigid update schedules, some receivers are persistent in their pursuit of scheduled refresh actions. Thus, energy may be repeatedly expended until an attempted update action is successfully accomplished.
When integrated into a wireless telephone, however, such rigid and relentless GPS data refresh schemes drain significant amounts of energy from the terminal's battery. In fact, without careful thought and innovation, the energy expended by the GPS receiver is likely to exceed by several-fold that expended by the baseline telephone alone.